the heart, have their cell-bodies in the bulb. It
thus appears that the entire control of the circulation is effected in a
reflex manner through the nerve centers in the bulb. These centers are
stimulated by conditions that relate to the movement of the blood through
the body.

*Respiration.*--Efferent fibers connect the different muscles of
respiration with a cluster of cell-bodies in the bulb, called the
_respiratory center_. This center together with the nerves and muscles in
question form an automatic, or self-acting, mechanism similar in some
respects to that of the heart. Through the impulses passing from the
respiratory center to the muscles, a rhythmic action is maintained
sufficient to satisfy the usual needs of the body for oxygen. The demand
of the body for oxygen, however, varies with its activities, and to such
variations the respiratory center alone is unable to respond. The
regulating factor in the respiratory movements has been found to be the
condition of the blood with reference to the presence of oxygen and carbon
dioxide. If the blood contains much carbon dioxide and little oxygen, it
acts as a strong stimulus to the respiratory center, causing it, in turn,
to stimulate the respiratory muscles with greater intensity and frequency.
On the other hand, if the blood contains much oxygen and little carbon
dioxide, it acts only as a mild stimulus. This explains how physical
exercise increases the breathing, since the muscles at work consume more
oxygen than when resting and give more carbon dioxide and other wastes to
the blood.

The respiratory center is also connected by afferent nerves with the
mucous membrane of the air passages. Irritation of the nerve endings in
this membrane causes impulses to pass to the center, and this leads, by
reflex action, to such modifications of the respiratory acts as sneezing
and coughing. There is also a connection between the cerebrum and the
respiratory center. This is shown by the fact that one can voluntarily
change the rate and force of the respiratory movements, and further by the
fact that emotions affect the breathing.

*Regulation of the Body Temperature.*--As explained in the study of the
skin (page 270), the nervous system regulates the body temperature by
controlling the circulation of the blood through the skin and the internal
organs. This is accomplished by stimulating in a reflex manner the muscles
in the walls of certain arteries. To prevent the body f